After an HTC Legend, the Android 2.1-based handset that's the manufacturer's follow up to the well-regarded Hero? You're going to have to get it from Vodafone, and if you want to one of the first to lay your mitts on it when it arrives on 2 April, you'll have to put your order in early.
Which you can now do, Vodafone said today …

Android 2.1

The Legend (like the Desire) is an Android 2.1 device. Vodafone don't have any choice but to make any proprietary changes they want to make to Android 2.1, then release it with that. It's not feasible to stuff a version of 1.6 onto a device designed for 2.1.

When the next Android version is released, that's a different matter. HTC or Vodafone might take their time to release an update specifically for this handset, in which case your only other option would be to root the device and install a hacked ROM (at your own risk).